1. Field of the Invention
The present invention relates to laser diodes, and more particularly to laser diodes having a narrow beam divergence and improved coupling efficiency with an optical system comprised of optical fibers, lenses and the like.
2. Description of the Prior Art
In recent years optical communication technology and optical information processing are playing major roles in various fields. Digital optical communication using optical fibers has made possible large increases in data communication densities, and optical disks and laser printers have produced a considerable expansion of the range of optical information processing applications. The progress of optical communications and optical information processing technology owes much to advances made in the laser diodes used as light sources. The small size and high efficiency that are features of laser diodes have brought these devices into widespread use, for example as light sources for compact disk systems, video disk systems and optical communication networks. In a laser diode the lasing action is generated by the injection of carriers into the P-N junction constituting the active layer. Recent advances in semiconductor technology such as MBE (molecular beam epitaxy) and MOCVD (metal-organic chemical vapor deposition) that make it possible to form epitaxial layers as thin as 1 nm or less, have led to the realization of laser diodes that use quantum well active layers less than 20 nm thick, with higher levels of efficiency and lower drive current requirements (see W. T. Tsang in "Semiconductors and Semimetals," vol. 24, pp 397, Ed. R. Dingle, Academic Press, San Diego (1987)).
Compared to gas lasers or ordinary solidstate lasers, the major feature of laser diodes is their small size and high efficiency. However, when incorporated into an actual system the laser beam has to be coupled with some form of optical system. Viewed from the system side the problem concerns the overall characteristics of the laser diode, including the coupling characteristics relative to the optical system. In general it is difficult to achieve high coupling efficiency owing to the beam having a wide beam divergence of 30 degrees or more and, rather than being isotropic, having a spatial asymmetry of 1:2 to 1:3 or more. Especially in recent years, when used as excitation sources for optical fiber amplifiers in communication systems or to excite solid-state lasers, high optical-coupling efficiency into a small region is becoming increasingly important. Also, in such applications as these in which high output power is required, a low coupling efficiency has to be offset by raising the optical output power of the laser diode accordingly, which lowers the reliability of the laser diode and, therefore, the reliability of the whole system.
The structure of prior laser diodes is a multilayer arrangement of semiconductor material formed by epitaxially growing the operating layers, including an optical waveguide layer structure, on a substrate. Such a structure results in a large variation in the index of refraction along a direction perpendicular to the substrate which, with the different layers, produces a strong optical confinement effect, so that when the light spot diameter is 1 .mu.m or less, diffraction gives rise to a large beam divergence angle of 20 to 30 degrees or more. In contrast, in a direction parallel to the substrate, in nearly all cases other than buried structures, the optical confinement is the result of the equivalent of changes in the index of refraction based on the differences in propagation constant produced by changes in the thickness of the layers, and as such the confinement effect is weak. Moreover, the waveguide structure is fabricated mainly using a photolithographic fabrication process that produces a waveguide width in the order of 2 to 5 .mu.m, and as a result the spot diameter increases to about the same size and, also, the weak diffraction causes a narrowing of the beam divergence angle to around 10 degrees or less. (See L. Figueroa in "Handbook of Microwave and Optical Components," vol. 3, Optical Components, pp 246-252, edited by K. Chang, Wiley-Interscience Publications, New York (1990)).
In order to improve the coupling efficiency between the diode and the optical system, first it is important to reduce the beam divergence along the direction perpendicular to the substrate. Generally this is done by reducing the thickness of the layers adjacent to the active region that have a high index of refraction, causing the light to penetrate into the low-refractive-index cladding regions around the beam and increasing the diameter of the light spot. However, the result of this is that the quantity of photons confined in the active layer is reduced by the amount by which the beam size is increased, thereby reducing the confinement factor and increasing the threshold current needed for oscillation to take place. In particular, in the case of quantum well active layer decrease in the confinement factor caused by the rapid saturation of the gain that accompanies an increase in injection carriers produces a considerable increase in the threshold current. While there is a method of compensating for this gain saturation by optimizing the number of quantum wells at around two or three, the threshold current increases with the increase in the number of quantum wells.
To overcome the drawbacks of the laser diodes described above, a laser diode has been proposed in which the cladding is a multilayer structure having a periodic refractive index differential (see M. C. Wu, et al., Applied Physics Letters, vol. 59, page 1046 (1991)). This PINSCH (for Periodic Index Separate Confinement Heterostructure) laser uses the same principle as a .lambda./4 shift DFB laser to prevent high-order mode oscillation from taking place even when there is an increase in beam diameter in the perpendicular direction. However, the drawback of this structure is that fabricating each of the layers making up the periodic index cladding structure requires highprecision control of layer composition and thickness, and in addition there has to be a gradual change in layer composition at the interfaces between layers in order to suppress increases in electrical resistance that can give rise to energy barriers in the multilayer structure. Another method involves reducing beam divergence by the addition to the waveguide region of layers having a lower refractive index (see T. M. Cockerill et al., Applied Physics Letters, vol. 59, page 2694 (1991)).
However, tests on beam divergence in the perpendicular direction produced a full-width at half maximum of 27 degrees and a threshold current density of 309 A/cm.sup.2 (with a resonator length of 780 .mu.m), inferior to an ordinary quantum well laser beam divergence angle of 35 degrees and threshold current density of 200 A/cm.sup.2. With this method, also, as the width of the diffracted far-field image pattern is reduced by raising the base of the optical waveguide mode, the intensity of the light confined in the cladding layers decays slowly, so that unless the cladding layers are quite thick, 2 or 3 .mu.m or more, the result, with the further absorption of the light by the buffer and cap layers beyond the cladding layers, is a large coupling loss. Moreover, the deep etching that has to be used when the active layer is overlaid with thick layers makes it difficult to fabricate lasers with graded-index and other types of waveguide structures.